The acetone-butanol-ethanol fermentation (ABE) has been known and practiced industrially for nearly 100 years. The biology, history and technology are reviewed in detail by Acetone-Butanol Fermentation Revisited, Microbial Reviews, Dec. 1986; 50(4); 484-524. The yield of total solvents, A+B+E, from carbohydrates are reported as about 32% w/w. There are two phases in the fermentation in which first acids are produced and subsequently solvents are produced from the acids. However not all of the acids, mainly acetic acid and butyric acid, are converted to solvents, so the final fermentation broth contains a number of co-products which need to be recovered or treated. Major co-products are also H2 and CO2. All of the biochemical pathways have been studied. Many attempts have been made to improve the yield and process economics of the process. A number of processes have been proposed to use the carbohydrates from biomass as the feedstock. However the main limitation is the low overall yield of the target solvents and thus poor economics which has caused all industrial production to be ended. It is a very mature technology.
Recently it has been proposed to use a hybrid process in which butyric acid is produced and then subsequently converted to butanol in a separate hydrogenation step. The advantage is that a fermentation organism can be chosen that produces butyric acid plus gaseous co-products such as Clostridium tyrobutylicum as proposed in US Patent Publication 2008/0248540. However this organism and most butyric acid bacteria also produce CO2 so that the carbon yield is low. The yield reported in the proposed process is 41% w/w on a theoretical basis.
While the ability to produce butanol and other solvents has been proven industrially and improved through further research, it would be desirable to find a process with higher yield which had better economics and could compete on an industrial basis.